1. Field of the Invention
The present invention relates to a process for making alkyl aromatic aldehydes and aromatic acids from alkyl aromatic compounds and to catalysts useful therein.
2. Description of the Related Art
Carbonylation of an alkyl aromatic compound to form an aldehyde can be carried out by a reaction generally referred to as the Gatterman-Koch reaction. Published in 1897, Gatterman and Koch described the direct carbonylation of various aromatic compounds by the use of carbon monoxide and hydrogen chloride in the presence of aluminum chloride and cuprous chloride (Gatterman, L. and Koch, J. A., Chem. Ber., 30, 1622 (1897)). The reaction was subsequently expanded to include other Lewis acids. Further, it was discovered that the cuprous chloride could be eliminated if the CO pressure was increased. A review of such reactions is set forth in Olah, G. A., "Friedel-Crafts and Related Reactions", Wiley-Interscience , N.Y., Vol. III, 1153 (1964).
U.S. Pat. No. 2,485,237, for example, describes replacing the hydrogen chloride and aluminum chloride catalyst combination with hydrogen fluoride and boron trifluoride. Further use of the HF--BF.sub.3 catalyst is described in U.S. Pat. No. 3,284,508 where the recovery of the fluorides is stated to be improved.
The HF--BF.sub.3 catalyst combination is sometimes modified to a two step process where a toluene--HF--BF.sub.3 complex is preformed and reacted with CO to form tolualdehyde. Afterward, make-up CO and optionally additional toluene are added to the reaction medium. An example of such a process is set forth in U.S. Pat. No. 3,948,998.
Other catalysts that have been reported for use in a Gatterman-Koch type carbonylation reaction include combinations of Lewis and strong Bronsted acids such as SbF.sub.5 --HF as is described in U.S. Pat. No. 4,218,403. The use of Bronsted superacids alone, such as fluorosulfonic acid or trifluoromethane sulfonic acid, were also reported to be effective catalysts. See for example Olah, G. A., Laali, K., and Farooq, O., J. Org. Chem., 50, 1483 (1985).
However, the catalysts used in a Gatterman-Koch carbonylation reaction are typically complexed with the aldehyde product. Thus, a stoichiometric amount of catalyst is "consumed" in the reaction. Further, in order to obtain the aldehyde product in a complex-free form, a separation step is needed. For instance, water can be added to a tolualdehyde--AlCl.sub.3 complex to obtain the aldehyde product in a complex-free form. However, this step also chemically alters and destroys the utility of the catalyst. Such a separation, which leads to a one time use of catalyst renders this process commercially unattractive as catalyst regeneration and recycle would be prohibitively expensive.
A method that includes catalyst recycling is proposed by Olah, G. A. et al., J. Am. Chem. Soc., 98:1, 296 (1976). Here, a modified Gatterman-Koch reaction that employs BF.sub.3 --HF as a catalyst complex is used to form the aldehyde. The reaction is carried out at low temperatures, typically from 0-20.degree. C., and with excess HF. The catalyst is separated from the aldehyde-catalyst complex by a distillation technique wherein the BF.sub.3 and HF are boiled off, condensed and returned to the carbonylation reactor.
While this method is useful, it is generally desirable to have a method that avoids the use of HF, a material which requires special containment and handling facilities. Also, it would be desirable to provide an alternate method for separating the aldehyde from the catalyst.